The invention on hand relates to an abrasive grain in accordance with the characterizing clause of claim 1, featuring a coating that contains at least one abrasive substance; a method for its production; and its use.
The use of abrasive substances in the production of abradants has been known, and they are frequently used in practice for abradants on a liner (abrasive bands and abrasive papers) as well as for bonded abradants (abrasive disks). The abrasive substances enter into chemical reactions during the grinding or, respectively, cause physical actions that have a positive impact on the grinding process and that lead to an increase of the serviceable life of the tool while simultaneously reducing or completely avoiding any thermal damages to the work piece. Thus, the intended use of the abrasive substances, similar to that of a high-temperature or high-pressure lubricant, is to reduce the friction between the abrasive grain particle and the work piece; to prevent a fusing of the grinding chip with the grain or with the work piece through a reaction with the fresh chip surface; to absorb the heat that is generated in the form of melting, evaporation, sublimation or dissociation heat; and to protect the cutting edges of the abrasive grain particle from any reaction with the material.
Particularly well-suited abrasive substances have turned out to be, e.g., halogenides (chloride of lead, cryolite, fluorite, potassium tetrafluoroborate, et al.); chalcogenides (pyrite, antimony sulfide, zinc sulfide, molybdenum sulfide, et al.); metals with low melting points (Zn, et al.); and high-pressure lubricants (graphite, et al.).
Usually, during the production of the abrasive, the abrasive filling material, in addition to other fillers, is added to the bonding agent mix that is then further processed into the abradant, together with the abrasive grain and other components of the mix (liquid resin, or others). In addition to the abrasive fillers, inactive fillers are frequently used, such as wood or rock dust, chalk, clays et al., that are usually used to fill plastics (for the purpose of saving resins); or compacting fillers, such as, e.g., glass fibers or others, that are used to increase the firmness of the connecting ridge, and/or fillers such as pumice stone or cork powder with the aid of which the self-sharpening properties of an abrasive disk are enhanced.
A disadvantage of this traditional method must be seen in the fact that the abrasive fillers are distributed evenly across the entire abrasive material. However, their abrasive effect is mostly required at a location where the actual abrasive action occurs, to wit: where the abrasive grain collides with the work piece, i.e. in the immediate vicinity of the abrasive grain. Thus, mole abrasive fillers are usually used than would be theoretically necessary which is of significance in so far as suitable substances are frequently very expensive and/or toxic.
In German patent publication DE 2 339 507, abrasive wheels are described that are based on abrasive grains featuring a sealing coating with material-active filler properties.
In each of those cases, it involves coatings that must be seen as specific for the production process and for the corresponding abrasive. Thus, the preferred objective is to make it possible to use hygroscopic, subliming or liquid substances as fillers.
Due to the danger of the formation of agglomerates or agglutination, such coated abrasive grains must be processed immediately. They show the same performance, and it is thus possible to reduce the use of abrasive fillers and to make it possible to use new, less critical fillers.
The invention, on the other hand, is based on the desire to develop abrasive grains that are universally usable for the most varied applications; that feature a coating that contains at least one abrasive filler; that can be stored, that do not have a tendency to agglomerate; and that are of a pourability that is sufficient for trouble-free processing.
One of the objectives of the invention is, above all, to increase the efficacy of the abrasive tool manufactured with the abrasive grain according to the invention which, indirectly, of course, reduces the amount of the abrasive filling material used which brings about additional ecological advantages.
This task is solved by first blending an abrasive grain intensively with a low-viscosity binder and then adding an abrasive filling material that provides the surface of the abrasive grail with a coating that contains an evenly distributed abrasive filler.
It was found that intensive blending of an abrasive grain with a low-viscosity binder and the subsequent adding of an abrasive filler will yield an even coating that can then be precured or precipitation-hardened through temperature treatment or through the addition of hardeners after which the thus treated abrasive grains can be stored even over a longer period without the formation of lumps or agglomerations. The abrasive grains feature excellent pourability and can be used, trouble-free, for the most varied applications. When the abrasive grain is used in abrasive tools, enormous performance increases have been observed.
In comparative tests it was further found that the abrasive grail according to the invention yielded clearly better abrasion outputs when used in abrasive tools as compared to an abrasive tool produced with the same amount of abrasive fillers on the basis of non-coated abrasive grains.
In the first step of the production of the abrasive grain according to the invention, the abrasive grains to be coated are blended with a binder until all grails are coated evenly with the binder.
Conventional abrasive grains such as corundum or SiC as well as the so-called super abrasive materials such as diamond or CBN can be used as abrasive grain. There are no restrictions with regard to grain size. However, the method is preferably used in the macro-grain range.
Inorganic as well as organic binder may be used as binders. Preferably, low-viscosity resins from the group of phenol resins arc used. Very good results have been achieved with base-condensed synthetic resins, or neutral phenol formaldehyde synthetic resins. Resins oil epoxy or polyurethane basis proved to be very suitable as well. But in addition to that, inorganic polymers from the group of silicates or phosphates are also well suited, with the use of aluminum phosphate given preference. The viscosity of the binder should be below 1,000 mPa*s, preferably below 700 mPa*s. The binder portion lies usually between 0.5 and 8 weight percents relative to the abrasive grain used. Particularly good results have been achieved when the binder portion is above 1 weight percent and does not exceed 5 weight percents.
For the blending, the usual mixing aggregates can be used. Using a heatable intensive mixer is advantageous, making it possible to carry out the blending with the binder; the admixing of the active substance, and the drying action (precuring) in one single step. The application of the coating can be done particularly evenly and elegantly in a fluidized bed where the mixing and precuring preferably occur in one process step as well.
In a second step the corresponding active substance is admixed with the abrasive grain coated with binder. One of the advantages of the method is the fact that for this purpose, all known abrasive fillers may be used. Preferably, sulfides, phosphates, carbonates, halogenides and/or complex compounds containing sulfides, phosphates, carbonates or halogenides from the group of elements Na, K, Ca, Mg, Al, Mn, Cu, Sn, Fe, Ti, Sb and/or Zn are used.
Surprisingly, very good results could be achieved with mixtures of calcium fluoride and tricalcium phosphate as well as manganese sulfate and lithium carbonate that were not known as abrasive fillers in this composition. In the case of both combinations, an additional advantage is the fact that they feature no or very low toxicity and that they are easy to handle.
The portion of abrasive filler is usually between 1 and 15 weight percent, preferably between 3 and 10 weight percent, in each case in relation to the amount of abrasive grain used. In order to achieve an even distribution of the filler in the processing of macro grains that cover a range of several millimeters to approximately 50 xcexcm, the mean particle size of the active material should be below 50 xcexcm, preferably below 20 xcexcm. In general, the particle size of the active material should be chosen in such a way that with an even coating of the abrasive grain with active material, the finished granulation will still be within the tolerance of the standard for grain size distribution of the uncoated original granulation. Therefore, for the coating of micro granulations, the abrasive fillers must be used in pigment form. It is particularly advantageous to use solutions of the corresponding abrasive fillers in the micro grain range which comprises grain sizes of approximately 80 xcexcm down to approximately 1 xcexcm.
As soon as the active material has been distributed on the abrasive grain particles moistened with binders, a partial or complete thermal hardening of the coating occurs during which the abrasive filler is firmly attached to the grain surface. The drying temperature itself varies, of course, depending on the binder being used. The preferred temperature range for the drying of liquid resins lies between 80 and 130xc2x0 C. The inorganic binders, on the other hand, must be processed at considerably higher temperatures. For example, a binder on aluminum phosphate basis requires roasting temperatures of at least 400xc2x0 C., preferably 700xc2x0 C. in order to achieve a hardening of the coating. All known drying or firing processes or, respectively, aggregates may be used. For many applications it will be advantageous to merely do a thermal precuring, thereby, at the same time, at least partially maintaining the reactivity of the binder in question which can then be used in the incorporation of the coated grain into another system.
If a polyurethane or epoxy resin is used as a binder, the precipitation hardening occurs through the addition of hardeners in accordance with the instructions of the manufacturer in question. In all cases, following the partial or complete hardening, a pourable and storable granulation will be the result that can be processed trouble-free, just like any untreated granulation.
The abrasive grain provided with an abrasive coating finds use in any kind of grinding or polishing tools. The abrasive grain according to the invention can be used particularly advantageously in synthetic-resin bonded abrasive tools (abrasive tools with a backing and abrasive disks or abrasive wheels). Surprisingly, in addition to the foregoing, it was determined within the framework of the tests that the incorporation of the grain into the synthetic-resin bonding per se will be clearly improved, meaning that the use of the abrasive grain according to the invention is not limited only to abrasive tools but makes sense in general with all synthetic-resin bonded products in which corundum is imbedded (laminates, lacquers, and the like).
In the following, the invention will be explained by way of, but without being limited to, a series of examples.